1. Field of the Invention
The present invention relates generally to the packaging and mounting of semiconductor devices in electronic circuits, and in particular, to a method and apparatus for engaging a module to a heat sink by providing a compressive force that redistributes and controls the compression of ball or column grid array modules.
2. Description of Related Art
Modern electronic modules include chip carriers, or substrates, onto which are mounted microchips and other circuit components, and which in turn, are mounted on printed circuit boards. During normal operating conditions, heat or thermal energy is generated from these microchips and other components. While some low power electronic components are able to dissipate heat generated from standard working conditions directly into the ambient, most require assistance to dissipate generated heat.
Heat sinks are often used to assist high power electronic components to absorb, channel away and dissipate the heat produced from such components during normal operating conditions. In so doing, thermal contact is required between the heat sink and the heat generating component for the effective use of such heat sink.
For example, the prior art illustrations of FIGS. 1A and 1B show a heat sink 10 coupled to an electronic module using a thermally conductive material, such as a thermal paste (not shown). The electronic module includes a chip 20 mounted on a chip carrier 30 and is connected to a printed wire board 50 via a solder interconnection array 40, such as a solder ball grid array (BGA) or a solder column grid array (CGA). During normal operations, the heat sink 10 assists in absorbing, channeling away and dissipating the heat produced from such electronic component via the thermally conductive connection there between.
To ensure and maintain the required heat transfer between the heat sink and the die, securing devices, which may include screws, springs, spring loaded screws, clips, clamps, and the like, are typically positioned at four locations such that they reside on an outside perimeter of the chip 20, chip carrier 30, and solder interconnection array 40 as is shown in FIGS. 1A and 1B.
Conventional techniques of securing the heat sink to the electronic component include positioning securing means, such as spring loaded screws 80, to a backside or underside of printed circuit board 50 whereby the securing means traverse through the board and into the underside of heat sink 10 as shown in FIG. 1A. Alternatively, securing means, such as spring loaded screws 85, may be positioned at a backside of the heat sink 10 and traverse there-through, into and through circuit board 50 and secured in position at the backside (underside) of the circuit board via bolts, as is shown in FIG. 1B. Referring to FIGS. 1A and 1B, the forces applied when tightening these conventional securing means 80, 85, whether the securing means be tightened from a backside of the heat sink or from a backside of the circuit board, result in a pressure between the heat sink and the die which ensures good heat transfer.
However, these conventional backside forces 90 undesirably apply high compressive forces to the extremity area of the electronic circuit that surrounds chip 20, chip carrier 30 and solder interconnection array 40 (BGA or CGA), and as such, are primarily applied to solder joints residing at an outside perimeter 42 of the solder interconnection array 40, as illustrated by the compression profiles 100 on the solder interconnection array 40 in FIGS. 1A and 1B. These high compressive forces on solder joints residing at the outside perimeter 42 of the solder interconnection 40 cause such solder joints to creep and thereby change their shape.
As solder balls or solder columns of the interconnection array 40 change shape or flatten, due to these compressive forces 90, the long-term reliability of the solder joint undesirably deteriorates. This is particularly true for the outer solder joints. The deterioration of solder joints results in reduced fatigue lifetime from thermal expansion mismatch among the various components. In extreme cases, creep flattening of the outer interconnections in the array 40 can result in shorting of adjacent interconnections, which represents catastrophic failure of the module itself. The affects of interconnection array 40 shape change or flattening of peripheral solder joints is even more prevalent in thin boards, as such boards bend easily under the action of the conventional backside applied forces 90, which in turn, even further increases pressures applied to the peripheral solder joints. These increased pressures enhance the risk of creep and the ensuing flattening and deterioration in product lifetime.
Accordingly, in order to avoid the problems of creep, flattening and deterioration in product lifetime, it is desirable to maintain the ball shape or column shape of the solder throughout solder interconnection array 40. This will enable the individual interconnections to efficiently accommodate strains generated from the thermal expansion mismatch between the chip carrier 30 and the circuit board 50 during normal working conditions.
Thus, a need continues to exist in the art for improved methods and apparatus for providing thermal and physical contact between a heat sink and chip, thereby enabling good heat transfer between the chip and heat sink, while minimizing bending of the circuit board. These methods and apparatus are preferably inexpensive, easy to assemble and/or disassemble for allowing reworkability or replacement of damaged or inoperative chips, and allow for numerous electronic components to be mounted on the module or on the circuit board.